[However, this] is insufficient for use on directories. As per RFC 3659 (http://tools.ietf.org/html/rfc3659#section-4.2), servers should return error 550 (File not found) if the command is issued on something else than a file, or if some other error occurred. For example, Filezilla indeed returns this string when using the ftp_raw command on a directory:

array(1) {
[0]=>
string(18) "550 File not found"
}

RFC 959 (http://tools.ietf.org/html/rfc959) dictates that the returned string always consists of exactly 3 digits, followed by 1 space, followed by some text. (Multi-line text is allowed, but I am ignoring that.) So it is probably better to split the string with substr, or even a regular expression.

note that project_t4 at hotmail dot com's example above doesn't work in general, though it works on his Win2K/Apache server; as far as I can tell there is no way to check over ftp whether a directory exists. This function's behavior given a directory name seems to be at least somewhat dependent on your OS, web server, or ftp server, I don't know which.